The Applicant has previously described a number of different wiping systems for wiping nozzle plates of pagewide printheads. Wiping interventions are typically required when printouts deteriorate in print quality. For example, the nozzle plate may become contaminated with flooded ink or the nozzle plate may become contaminated with particulates (e.g. paper dust), which require periodic removal to maintain optimal print quality.
Printhead wiping may be either along a longitudinal extent of the printhead (see, for example, WO2013/059853) or transversely across the printhead (see, for example, WO2011/143699). Usually, printhead wiping is sufficient to recover print quality; however, periodic wiping is undesirable in some types of printers.
In a digital inkjet press, a web of print media is fed continuously past one or more fixed printheads at very high speeds (e.g. greater than 30 inches per second). An example of a digital inkjet press is described in U.S. Pat. No. 8,616,678, the contents of which are incorporated herein by reference. Since the web cannot be broken, in order to wipe printheads in a digital inkjet press of the type described in U.S. Pat. No. 8,616,678, the printheads must be lifted away from the web of print media and positioned adjacent corresponding wipers. Once the printheads have been wiped, the printheads are lowered back to a printing position so that printing may be resumed. This wiping operation typically takes 20 seconds or more and necessarily requires the web feed mechanism to be stopped during wiping; otherwise, dozens of feet of the print media are wasted for each wiping intervention.
However, stopping the web feed mechanism is undesirable in a high speed press. Web feed mechanisms are designed to run continuously for long periods—stopping the press for frequent maintenance interventions has a very significant effect on overall print speeds, interferes with the operation of downstream web cutters, and ultimately affects the economic feasibility of installing the digital web press.
At high print speeds, a common type of print defect is caused by condensation of water vapor on parts of the printhead assembly. Referring to FIG. 1, there is shown a printer 1 wherein a web 3 of print media is fed over an apertured platen surface 6 past a printhead 7 in the direction indicated by the arrow D at a speed of about 55 inches per second (ips). The region 10 immediately downstream of the printhead 7 has a relatively high humidity. Sources of water vapor in the region 10 include: hot ink droplets in flight which are ejected from the printhead 7, ink droplets striking the web and evaporating; and thermal ink ejection processes in the printhead 7, whereby aqueous ink is superheated in firing chambers and vapor bubbles are vented through inkjet nozzles.
Water vapor in the region 10 will tend to condense on any cool surface in the vicinity. Although the printhead 7 itself is hot during printing, the printhead assembly 12, on which the printhead is mounted, is relatively thermally isolated from the printhead and therefore remains relatively cool. Hence, relatively cool parts of the printhead assembly 12 downstream of the printhead 7 collect condensation 13 during printing, and this condensation accumulates over time. In a Memjet® printhead assembly 12, shown schematically in FIG. 1, condensation 13 usually accumulates on the lower surface of an encapsulant material 14 positioned immediately downstream of the printhead 7. The encapsulant material 14 is typically a polymer, which encapsulates wirebond connectors supplying power and data to the printhead.
Condensation on the surface of the printhead assembly 12 is problematic for print quality in two respects. Firstly, the condensate 13 may drip from the surface directly onto the web 3 and cause mottling of the printout. Secondly, and more significantly, the condensate 13 may migrate back onto the nozzle plate 15 of the printhead via capillary action. If water droplets reach the nozzle plate 15, they are quickly sucked into inkjet nozzles due to backpressure in the ink delivery system supplying ink to the printhead. Hence, inkjet nozzle chambers and ink supply channels in the printhead 7 may become filled with water instead of ink. If a group of nozzles print water instead of ink, this has a severe effect on print quality until the water is cleared from the printhead via a number of droplet ejections. At high print speeds, the effects of condensation are typically manifested in streaks in the printout.
It would therefore be desirable to provide a means for mitigating the effects of condensation during high speed printing. It would be further desirable to provide a method of wiping a printhead assembly, which does not require either stopping or breaking the web.